The present invention relates generally to radio or wireless communications and, more particularly, relates to a method and apparatus for setting the initial transmit power and data rate in the forward link of a code division multiple access (CDMA) wireless communications system.
In conventional CDMA systems, forward link (base station to mobile station) transmit power and data rate control is a base station function that is activated each time one or multiple traffic connections are established. The traffic channel data rate and the traffic channel transmit power are selected such that (1) the base station forward link residual capacity does not dip below a desired threshold; and (2) the desired received traffic channel signal-to-noise ratio is approximately achieved at the mobile station antenna connector. Closed loop power control is then used to adjust the traffic channel transmit power and to compensate for sub-optimum open loop estimation and changing channel conditions.
Conventional approaches treat traffic channel data rate and power control separately and in a heuristic fashion. With regard to power control, the initial forward link traffic channel transmit power is set to a default value at call setup and during hard handoff. If this default value is set too low, the mobile station traffic channel received signal-to-noise ratio will be insufficient to guarantee a reliable traffic channel connection, which may result in a call being dropped or in service quality degradation. For this reason, the default value is usually set at a higher level than is required on average in order to provide a reliable connection even in the worst or near-worst case channel conditions.
This approach to power control has several shortcomings. By setting the initial traffic channel transmit power to a high default level, the mobile station traffic channel received signal-to-noise ratio will frequently exceed the required level and forward rink capacity will be unnecessarily consumed. Moreover, a high default value increases the total transmit power peak to average ratio, leading to inefficient use of the base station high power amplifier. Where multiple traffic channel connections must be established nearly simultaneously, and where high rate data users consume a significant amount of the total forward link capacity, the transmit power peak to average ratio increases. A reduction in the maximum average nominal transmit power may be required to avoid damage to the base station high power amplifier and/or short term operation in the non-linear region of the high power amplifier, which may lead to increased spurious emission levels. Reducing the maximum average nominal transmit power, however, reduces the efficiency of the amplifier.
The transmit data rate is chosen based on the service option to be connected and on the base station residual forward link (base station to mobile station) capacity. This approach to setting the data rate is also problematic. If the selected data rate is too high for a given initial traffic channel transmit power and residual forward link capacity, the mobile station may not be able to acquire the forward link traffic channel. If the traffic channel is acquired, the power allocated to the traffic channel may exceed the residual forward link capacity and thereby cause overload.
The present invention uses forward link load measurements and mobile station channel measurements to allow the base station to jointly select the initial traffic channel transmit power and data rate. Using this approach, the mobile station can reliably acquire the forward link, forward link capacity consumption is minimized, base station total transmit power peak to average ratio is reduced, and the power allocated to the traffic channel is no more than the residual forward link capacity.
In one embodiment of the invention, a CDMA wireless communications system that controls the initial power and data rate settings during forward link establishment of a traffic channel is provided. The system comprises a base station controller, one or more base stations that report a current forward link load estimate to the base station controller, and a mobile station that reports a received pilot channel signal-to-noise ratio to the base station controller. The base station controller sets the number of traffic channel connections and the data rate to be allocated for the traffic channel connections. The base station controller also sets a target traffic channel signal-to-noise ratio based on the data rate and the number of traffic channel connections, and computes the initial traffic channel power based on estimated forward link channel characteristics, the target traffic channel received signal-to-noise ratio, the base stations"" load, the number of traffic channel connections, and the traffic channel data rate.
The present invention also provides a method for forward link open loop power and rate control in a CDMA system. The method comprises the following steps:
(a) a mobile station reports a received pilot channel signal-to-noise ratio,       Pilot    ⁢                  E        c                    I        o              ,
xe2x80x83for all base stations in sight to a base station controller;
(b) the base stations report a current forward link load estimate,             Pilot      ⁢              xe2x80x83            ⁢              E        c                    I      or        ,
xe2x80x83to the base station controller;
(c) the base station controller sets a number of traffic, channel connections, N, and a traffic channel data rate, Rb, to be allocated for the traffic channel connections;
(d) the base station controller computes a target traffic channel signal-to-noise ratio,       Traffic    ⁢                  E        b                    N        t              ,
xe2x80x83based on the data rate and the number of traffic channel connections; and
(e) the base station controller computes the initial traffic channel transmit power relative to the pilot channel power,             Traffic      ⁢              xe2x80x83            ⁢              P        TX                    Pilot      ⁢              xe2x80x83            ⁢              P        TX              ,
xe2x80x83based on estimated forward link channel characteristics, the target traffic channel received signal-to-noise ratio, the base stations"" load, the number of traffic channel connections, and the traffic channel data rate.
In one implementation of this method, the base station controller computes the initial traffic channel power as:             Traffic      ⁢              xe2x80x83            ⁢              P        Tx                    Pilot      ⁢              xe2x80x83            ⁢              P        Tx              =                    Traffic        ⁢                              E            b                                N            t                                                            R            c                                R            b                          ·                              ∑                          i              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                                                                          Pilot                    ⁢                                          xe2x80x83                                        ⁢                                          E                                              c                        ,                        i                                                                                                  I                                          or                      ,                      i                                                                      ·                Pilot                            ⁢                                                E                                      c                    ,                    i                                                                    I                  o                                                                                                      Pilot                  ⁢                                      xe2x80x83                                    ⁢                                      E                                          c                      ,                      i                                                                                        I                                      or                    ,                    i                                                              -                                                                    1                    n                                    ·                  Pilot                                ⁢                                                      E                                          c                      ,                      i                                                                            I                    o                                                                                            .  
A method for forward link power and rate control during hard handoff in a CDMA system is also provided. The method comprises the following steps:
(a) a mobile station searches for pilot signals on a target frequency and reports the strength of the pilot signals above a predetermined threshold to a base station controller, wherein the reported pilot signals form a requested target set;
(b) base stations corresponding to the pilot signals in the requested target set report the current total transmit pilot channel signal-to-noise ratio;
(c) the base station controller sets a tentative granted set equal to the requested target set;
(d) the base station controller sets a target data rate to one in use by a connected service option;
(e) the base station controller sets the required traffic channels total signal-to-noise ratio to a predefined value based on the tentative granted set and the target data rate;
(f) the base station controller computes the required initial traffic channel power based on the tentative target set, the target data rate, the total transmit pilot channel signal-to-noise ratio and the required traffic channels total received signal-to-noise ratio;
(g) the base station controller determines whether a current base station residual capacity is above a desired threshold for the tentative granted set;
(h) if the residual capacity is above the desired threshold, the base station controller sets a granted set equal to the tentative granted set and commands the mobile station to initiate hard handoff; and
(i) if the residual capacity is below the desired threshold, the base station controller educes the tentative granted set and the method returns to step (f).
In another embodiment of the present invention, a method for setting the data burst transmission rate on common control channels during soft handoff is provided. The method comprises the following steps:
(a) a base station controller receives a message to be delivered to a mobile station;
(b) the base station controller pages the mobile station on the primary common channels of a set of base stations that are serving the mobile station;
(c) the mobile station reports its current location and the strength of pilot signals above a predetermined threshold to the base station controller, wherein the reported pilot signals form a requested common control channels handoff set;
(d) the base stations corresponding to the pilot signals in the requested target set report the current total transmit pilot channel signal-to-noise ratio to the base station controller;
(e) the base station controller sets a tentative data rate equal to a maximum data rate;
(f) the base station controller sets the required common control channels total signal-to-noise ratio to a predefined value based on the tentative data rate and handoff set size;
(g) the base station controller computes the required common controls channel power based on the tentative data rate, the handoff set size, the total transmit pilot channel signal-to-noise ratio and the required common control channels total received signal-to-noise ratio;
(h) the base station controller determines whether a current base station residual capacity is above a desired threshold for all base stations in the handoff set;
(i) if the residual capacity is above the desired threshold, the base station controller sets a granted data rate equal to the tentative data rate and transmits the message on secondary common control channels of the base station in the handoff set; and
(j) if the residual capacity is below the desired threshold, the base station controller sets the tentative data rate to the next lower supported data rate and the method returns to step (g).
Objects and advantages of the present invention include any of the foregoing, taken alone or in combination. Further objects and advantages will be apparent to those of ordinary skill in the art, or will be set forth in the following disclosure.